Create an io.ReaderAt from io.Reader - go

Question

Is there an implementation of io.ReaderAt that can be created from an implementation of io.Reader without first being read into a []byte or string?

Answer 1

Something like the below. Note bytes.Reader implements the ReadAt(...) method/function: https://golang.org/pkg/bytes/#Reader.ReadAt. So the line bytes.NewReader is esssentially what you are looking for.
Getting a bytes.Reader:
var ioReader io.Reader
...
buff := bytes.NewBuffer([]byte{})
size, err := io.Copy(buff, ioReader)
if err != nil {
return err
}
reader := bytes.NewReader(buff.Bytes())
// Do something with `reader`

Answer 2

Yes, this is possible. As mentioned in my comment above, the implementation is limited in that you cannot seek backward nor can you re-read a section that has already been read.
Here is a example implementation:
type unbufferedReaderAt struct {
R io.Reader
N int64
}
func NewUnbufferedReaderAt(r io.Reader) io.ReaderAt {
return &unbufferedReaderAt{R: r}
}
func (u *unbufferedReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
if off < u.N {
return 0, errors.New("invalid offset")
}
diff := off - u.N
written, err := io.CopyN(ioutil.Discard, u.R, diff)
u.N += written
if err != nil {
return 0, err
}
n, err = u.R.Read(p)
u.N += int64(n)
return
}
Example usage:
s := strings.NewReader("hello world")
var b [5]byte
ura := NewUnbufferedReaderAt(s)
if _, err := ura.ReadAt(b[:], 0); err != nil {
panic(err)
}
fmt.Printf("%s\n", b[:]) // prints "hello"
/*
if _, err := ura.ReadAt(b[:], 0); err != nil {
panic(err) // panics
}
fmt.Printf("%s\n", b[:])
*/
if _, err := ura.ReadAt(b[:], 6); err != nil {
panic(err)
}
fmt.Printf("%s\n", b[:]) // prints "world"